SUMMARY This project seeks to discover and describe mechanisms of virus assembly, ultimately at near-atomic resolution and including an understanding of the dynamic and energetic aspects of virus capsid maturation. Objects to be studied include the head of bacteriophage HK97, with particular interest in capsid assembly, capsid maturation, and control and mechanism of covalent crosslinking; phage lambda tails, with particular interest in length determination and the role of assembly chaperones; phage lambda long tail fibers, with particular interest in the role and mode of interaction of an assembly chaperone; and the capsids of a group of phages with unusually large capsids, with particular interest in how large capsid sizes are determined accurately. This work will enhance our understanding of virus life cycles and of molecular machines. The group of viruses under study shares many of the features of its assembly and maturation with animal viruses, especially including Herpesvirus, so these studies have the potential to enhance our understanding of human pathogens.